Read
Ao no Exorcist Manga
Online

This world consists of two dimensions joined as one, like a mirror. The first is the world in which the humans live, Assiah. The other is the world of demons, Gehenna. Ordinarily, travel between the two, and indeed any kind of contact between the two, is impossible. However, the demons can pass over into this world by possessing anything that exist within it. Satan may be the king of demons, but there's one thing he doesn't have, and that's a substance in the human world which is powerful enough to contain him! For that purpose he created Rin, his son with a human woman, but will his son agree to his plans? Or will he become something else...like an exorcist?